Mechanical response of Q2 loess stratum surrounding a hydraulic tunnel under dry-wet cycles

Abstract

Understanding the mechanical response of Q2 loess subjected to dry-wet cycles (DWCs) is the premise for the rational design of a hydraulic tunnel. Taking the Hanjiang-to-Weihe south line project in China as the research background, the microstructure evolution, strength degradation and compression characteristics of Q2 loess under different DWCs were investigated, and the fluid-solid coupling analysis of the hydraulic tunnel was carried out using the FLAC3D software. The amplification effect of tunnel surrounding soil pressure (SSP) and its influence on the long-term stability of the tunnel under different DWCs were obtained. The results showed that the pore microstructure parameters of the undisturbed and remolded loess basically tend to be stable after the number of DWCs exceeds 3. The porosity of Q2 loess is increased by 26%. The internal friction angle and cohesion of Q2 loess are decreased by 35% and 31%, respectively. The vertical strain of Q2 loess is increased by 55% after considering the DWCs. After the DWCs stabilized, the SSP ratio is increased between 10% and 25%. With the increase in buried depth of the tunnel, the SSP ratio is increased by 8%–10%. The SSP is reduced from 8% to 16% by the rise in groundwater level. As the number of DWCs increases and the burial depth of the tunnel decreases, the distribution of SSP becomes progressively more non-uniform. Based on the amplification factor and the modified compressive arch theory, the SSP distribution model of loess tunnel was proposed, which can be preliminarily applied to the design of supporting structures considering DWCs.

Keywords

• Q2 loess
• Hydraulic tunnel
• Surrounding soil pressure (SSP)
• Dry-wet cycles (DWCs)
• Amplification effect